Mental health experts who evaluated the man accused of killing 12 people in a Colorado movie theater last year have offered a diagnosis that bolsters an insanity plea in the case, his lawyers said at a hearing here on Monday.

“We now have a diagnosis that’s complete,” Daniel King, a defense lawyer for the suspect, James E. Holmes, said in court. “We now have an opinion by qualified professionals.”

Mr. Holmes, 25, a former graduate student in neuroscience, faces 166 counts of murder, attempted murder and weapons charges in the July 20 shooting during a midnight premiere of the Batman movie “The Dark Knight Rises” at an Aurora movie theater. Officials say he slipped out of an emergency exit shortly after the movie began, sheathed himself in commando-style gear and then returned through the same door to spray the sold-out crowd with gunfire.

Mr. Holmes’s lawyers made a long-expected move on Monday to change his plea to not guilty by reason of insanity. At an arraignment in March, a judge entered a straightforward not guilty plea on Mr. Holmes’s behalf after his lawyers said they were not ready to enter a plea.

An insanity plea would mean that the case against Mr. Holmes — and questions of whether he should be put to death if he is convicted — will rest largely on his mind-set on the night of the shooting.

Defense lawyers have suggested several times in court that Mr. Holmes is mentally ill, but they have not provided details. A judge’s order that has been in place for months has prevented both sides from discussing the case outside of court.

Mr. Holmes had seen a psychiatrist at the University of Colorado Denver before the shootings, and she had warned the campus police that Mr. Holmes had threatened her and had made homicidal statements. Weeks before his rampage, Mr. Holmes also sent a text message to a classmate asking about “manic dysphoria” — a psychiatric condition that combines the symptoms of mania and depression — and warning her to stay away from him because he was “bad news.”

Prosecutors and the police have characterized Mr. Holmes’s actions as methodical and deliberate. They say that he spent weeks assembling an arsenal of guns, ammunition and explosives and that he scouted the theater, the Century 16 multiplex, weeks in advance, taking photographs of the exterior and an inside door jamb. They also said he laid explosive booby traps at his Aurora apartment designed to kill or maim anyone who came looking for him.

The judge in the case, Carlos A. Samour Jr., said Monday there was good cause for Mr. Holmes to change his plea, but he stopped short of allowing it. Mr. Holmes’s lawyers have raised questions about a thicket of issues regarding the legal implications of the court-ordered examinations that Mr. Holmes will have to undergo to pursue an insanity defense. Those disputes have to be resolved first.

The judge is expected to formally accept Mr. Holmes’s insanity plea later this month.